Portable apparatus is more and more popular, and therefore capacitor charger it uses receives more attentions than ever. FIG. 1 shows a diagram of a conventional capacitor charger 100 implemented with ring choke converter (RCC) circuit, in which a transformer T1 connected with an input voltage Vin transforms a primary current IP to a secondary current IS that flows through a diode D1 to charge a capacitor C1 to generate an output voltage Vout at the output 112 of the charger 100 to supply for a load RL. To control the current IP or IS, a power transistor Q1 has its collector 104 and emitter 108 connected to the primary winding N1 of the transformer T1 and ground, respectively, and resistors R1 and R2 are connected between the input 102 of the charger 100 and another primary winding N3 of the transformer T1 for the bias to the base of the transistor Q1. Although the capacitor charger 100 is cheaper, the transformer T1 has a more complicated structure to construct the circuit, and the power transistor Q1 causes higher switching loss. In addition, there are too many parameters effective to the current IP, for example the input voltage Vin, transformer T1 and power transistor Q1, for the current IP to be precisely controlled. Furthermore, the maximum of the current IP has to be set much smaller than the base current Ib of the power transistor Q1, and much longer charging time is thus needed.
FIG. 2 shows a diagram of another conventional capacitor charger 200, in which a transformer T1 connected with a battery voltage Vbat transforms a primary current IP to a secondary current IS to charge a capacitor C4 for generating an output voltage Vout. To control the current IP or IS, a current sense resistor RS1 is connected between a transistor Q1 and ground, another current sense resistor RS2 is connected between the secondary winding N2 of the transformer T1 and ground, and operational amplifiers A1 and A2 are used to amplify the voltage drops across the resistors R1 and R2 to generate detector signals S1 and S2 for the inputs R and S of a flip-flop 206, respectively, so as to generate a switch signal S3 for switching the transistor Q1 through a driver 208. In the capacitor charger 200, the primary current IP and secondary current IS of the transformer T1 are directly detected by the current sense resistors RS1 and RS2 to determine the maximum and minimum of the primary current IP. However, for not affecting the operations of the charger 200, only much small resistors can be used for the current sense resistors RS1 and RS2, and therefore the voltage drops across the resistors RS1 and RS2 are very small. As a result, the currents flowing through the resistors RS1 and RS2 cannot be precisely detected, and it is also easily to introduce spikes on the primary current IP out of the maximum current that the battery could supply for the charger 200. Furthermore, the charging current IP of the charger 200 is not easy to set.
FIG. 3 shows an application of yet another conventional capacitor charger 300 for a flash lamp module 306, in which a transformer 304 has a primary winding L1 connected between an input voltage Vbat and a transistor 308 to transform the primary current IP to a secondary current IS by switching the transistor 308, so as to charge a capacitor C1 for supplying to the flash lamp module 306. To control the charging current IS, resistors R1, R2 and R3 are connected in series between the output 316 of the charger 300 and ground to detect the voltage on the capacitor C1 by voltage dividing to provide a feedback signal VFB to a controller integrated circuit chip 302, so as to stop charging the capacitor C1 when the voltage on the capacitor C1 reaches its threshold. In the capacitor charger 300, it is the maximum duty of the transistor 308 to be used to determine the charging current IP, and unfortunately, which is easy to have the transformer T1 to be magnetically saturated and inefficiently operated. Moreover, the input voltage Vbat is supplied by battery, and it will decade as time goes by, resulting in longer and longer charging time for the capacitor C1 to reach the same voltage.